The invention relates to a semiconductor device comprising integrated circuit elements realized in a stack of layers on a substrate and comprising means for preventing pollution of the circuit elements and of the substrate by hydrogen originating from their environment.
The invention finds its application in the semiconductor manufacturing industry, including the manufacture of integrated circuits which are incorporated in hermetically sealed modules such as transmission modules used in the field of telecommunications.
A semiconductor device comprising means for preventing the pollution of circuit elements by hydrogen is known from patent application JP no. 64-207266 of Oct. 8, 1989 published Mar. 27, 1991 (FUJITSU LTD.; TAKAYUKI OBA). This patent application describes means for preventing the hydrogen from diffusing into the semiconductor substrate, thus improving the reliability of a semiconductor device. These means consist of a layer which absorbs the hydrogen contained in insulating layers.
The use of these means involves the realization of a composite film comprising a metal layer made of a metal which absorbs the hydrogen, forming a hydride, arranged in sandwich fashion between two insulating layers of silicon nitride (SiN). This composite film is deposited on the upper surface of a semiconductor device which comprises a substrate and a circuit element formed on said substrate. The insulating layer, which contains nitrogen (N), such as the silicon nitride (SiN) layer, contains much hydrogen. The metal layer which is sandwiched between the insulating layers absorbs this hydrogen. A suitable metal for absorbing the hydrogen is palladium. For example, the composite film is formed by the metal layer of palladium sandwiched between two layers of silicon nitride. The result of this is that the hydrogen present within a protective insulating layer formed at the upper surface of the semiconductor device and the hydrogen present within an intermediate insulating layer can be captured by the sandwiched metal layer. The diffusion of the hydrogen into the substrate is thus suppressed and the generation of hot electrons in the substrate can be reduced to a minimum.
It is an object of the invention to provide means for eliminating the hydrogen originating more in particular from the environment of an integrated semiconductor device when this semiconductor device is protected by a housing. It is known to those skilled in the art that the metal parts of protective housings of xe2x80x9cchipsxe2x80x9d or integrated semiconductor devices are subject to aging, during which they evolve especially hydrogen. If the semiconductor device is realized on a substrate of a material from group III-V, for example gallium arsenide, the hydrogen will react with the substrate material and act as a neutralizer of charges, either at the surface or inside the material itself. If the semiconductor device comprises a stack of epitaxial layers and one of these materials is from the group III-V, the hydrogen will act as a charge neutralizer at the interfaces of the layers or inside the layers. The diffusion of hydrogen thus has a major detrimental effect on the surface charges, the interface charges, and on the doping profiles of the substrate and the layers.
This phenomenon is generally referred to as hydrogen poisoning. Its effect may become apparent at temperatures of the order of 100xc2x0 C. and higher. It should be borne in mind that the operational standards in the field of integrated circuits may demand that the circuits operate at temperatures of up to 125xc2x0 C., or even up to 200 or 300xc2x0 C. without any deterioration in performance, depending on the envisaged application. In particular, a circuit included in a module which is launched in a telecommunication satellite must be capable of operating at temperatures of the order of 300xc2x0 C. It is found that from the moment when a semiconductor device of the type described above is enclosed in a protective housing, this semiconductor device exhibits a deterioration in its performance within a very short time, of the order of a few hundred hours, or even just a few hours, depending on the temperature at which it is made to operate. Experiments have shown that the use of a metal layer made of a hydrogen-absorbing metal sandwiched between two insulating layers so as to form a composite film covering the integrated circuits has a detrimental influence on the integrated circuit because this composite film constitutes a strong parasitic capacitance which downgrades the performance levels of all elements of the integrated circuit even at room temperature. Such a layer realized in accordance with the cited patent application should accordingly be steered clear of altogether.
The cited document teaches that the hydrogen originates from the insulating layers themselves. Experiments have accordingly been carried out for verifying the presence of this problem and these experiments have shown that the effect produced thereby is negligible. Indeed, it is the hydrogen originating from the environment of the semiconductor device, and in particular from the housing, which has a very important detrimental effect.
According to the invention, this detrimental effect is suppressed by means of a device provided with means for preventing pollution of the circuit elements via a layer of material which absorbs hydrogen and which forms a pattern integrated with the circuit elements.
An advantage of the device according to the invention is that the detrimental effect of the hydrogen on the integrated circuit elements is no longer present once this device has been enclosed in a housing. The deterioration in performance of the integrated circuits can no longer be observed in time. It follows from this that the hydrogen contained in the integrated circuit itself is not the major factor of deterioration as taught in the cited patent application. But whatever the cause, the means according to the invention also solve the problem known from the cited patent application.